Identify the type and then write the balanced chemical equation for the reaction described below

1. Identify the type and then write the balanced chemical equation for the reaction described below “aluminum oxide is heated”

2. Ch. 9 Stoichiometry 9.1/9.2 Stoichiometric Calculations

3. Stoichiometry • deals with mass relationships between reactants and products in a chemical reaction

4. Moles A ⇄ Moles B • to get mole ratios, you must have a balanced chemical equation • tell us the ratio of the number of particles needed of each type to produce each type • used to convert between moles of one substance to moles of another substance

5. Example Al2O3(l)  Al(s) + O2(g) • If the reaction begins with 13.0 mol of Al2O3, how many moles of Al will be created?

6. Example • If the reaction ends with 28.0 moles of O2, how many moles of Al2O3 did you begin the reaction with? • How many moles of Al did you also end with?

7. Grams A ⇄ Moles B • use molar mass and mole ratio • grams A  moles A using molar mass • moles A  moles B using mole ratio OR • moles A  moles B using mole ratio • moles B  grams B using molar mass

8. Example 2 • If you began the reaction with 2.00 g of Al2O3, how many moles of O2 will you end up with? • g  mol using molar mass • mol  mol using mole ratio

9. Example 2 • If the reaction ended with 23.7 mol Al, how many grams of O2 would it also end with? • mol  mol • mol  grams

10. Grams A ⇄ Grams B • use molar mass twice • grams A moles A using molar mass • moles A  moles B using mole ratio • moles B  grams B using molar mass

11. Example 3 • If you begin the reaction with 14.45 g Al2O3, how many grams of Al would it end with? • grams Al2O3  mol Al2O3 using molar mass • mol Al2O3  mol Al using mole ratio • mol Al  grams Al using molar mass

12. Example 3 • If the reaction ended with 21.9 g of O2, how many grams of Al2O3 did the reaction begin with? • grams O2  mol O2 using molar mass • mol O2  mol Al2O3 using mole ratio • mol Al2O3  grams Al2O3 using molar mass

13. A sandwich consists of two slices of bread, 3 slices of meat, and one slice of cheese. For each of the following amounts, determine the number of sandwiches that can be made and what is left over: • 6 bread, 10 meat, 4 cheese slices • 10 bread, 6 meat, 8 cheese slices • 25 bread, 15 meat, 12 cheese slices

14. 6 bread, 10 meat, 4 cheese slices • 3 sandwiches • 0 bread, 1 meat, 1 cheese • 10 bread, 6 meat, 8 cheese slices • 2 sandwiches • 6 bread, 0 meat, 6 cheese • 25 bread, 40 meat, 12 cheese slices • 12 sandwiches • 1 bread, 4 meat, 0 cheese

15. Ch. 9 Stoichiometry 9.3 Limiting Reactant and Percent Yield

16. Why is there a limiting reactant? • a reaction rarely has exactly the right amount of each reactant • usually have some left over • limiting reactant • reactant that limits the amount of product created • always completely used up • excess reactant • reactant not completely used up

17. When do you have to find a LR? • whenever two amounts of reactants are given in a problem • when only one amount of reactant is given in a problem, then the other is assumed to be in excess

18. Finding Limiting Reactant • How much do you have available? • Find the number of moles of both reactants • Figure out how much you need of B if you use up all of A • Convert moles A to moles B using mole ratio • You may start with either reactant • Determine whether you will have enough • If you don’t have enough of B, then B is LR • If you do have enough of B, then A is LR

19. Example 1 • The reaction begins with 2.51 g of HF and 4.56 g of SiO2. What is the limiting reactant and the excess reactant? • Write the balanced chemical equation • SiO2(s) + 4HF(g)  SiF4(g) + 2H2O(l)

20. Example 1 • Find the number of moles available of each reactant:

21. Example 1 • If we use up all of the HF, how much SiO2 will we need to go with it? • Do we have enough SiO2? • 0.0759 mol available > 0.0313 mol needed • YES- there will be some left over • Limiting Reactant : HF

22. How much ER is left over? • Find out the number of moles of ER used up in the reaction • convert moles of LR to moles of ER using mole ratio • Subtract that amount from the moles you started with

23. Example 1 • How many grams of SiO2 will be left over? • Find the moles of SiO2 used up. • Subtract that from the moles started with • 0.0759 mol available - 0.0313 mol needed = .0446 moles SiO2 left over • Convert moles to grams using molar mass

24. How much of the product can be formed? • Start conversion with moles of limiting reactant. • Convert to moles of product using mole ratio • Convert to grams if requested using molar mass.

25. Example 1 • How many grams of water could be formed? • Convert moles of HF to moles of water • Convert moles to grams using molar mass.

26. Example 2 • A reaction was done with 36.8 g C6H6 and 41.0 g of O2. • Write the balanced chemical equation

27. Example 2 • What is the limiting reactant? • Find the number of moles of each • Convert one to the other • Compare to amount available • don’t have enough so O2 is LR and C6H6 is ER

28. Example 2 • Find the amount of ER left over • Find the moles ER used • Subtract from moles starting

29. Example 3 • If the reaction below begins with 51.03 grams of Fe and 37.5 grams of oxygen, what is the limiting reactant? 4Fe(s) + 3O2(g)  2Fe2O3

30. Example 3 • How many grams of oxygen will be left over after the reaction? • How many grams of iron (III) oxide can be formed?

31. Why percent yield? • Usually, not all the product possible is actually formed. • because of error in lab procedure • theoretical yield • maximum amount of product possible • is calculated using limiting reactant • actual yield • the measured amount formed in lab reaction • always less than or equal to theoretical yield

32. Example 1 • If the reaction actually produced 1.02 grams of water, what is the percent yield?

33. Example 2 • If 72.0 g of C2H2 reacts with an excess of Br2 and 729 g of the product is recovered, what is the percent yield? C2H2 + Br2 CHBr2CHBr2 • Do we need to find the LR first? • No- they tell you that the C2H2 is LR • How do we find the theoretical yield? • start with the LR and convert to product

34. Example 2 • Find the theoretical yield: • Calculate the percent yield:

35. Example 3 • If the percent yield of the reaction below is 73.8% and the reaction began with 24.3 g of CaO, how many grams of Ca(OH)2 were created? CaO + H2O  Ca(OH)2 • know percent yield • trying to find actual yield • need to calculate the theoretical yield first

36. Example 3 • Find the theoretical yield from LR first: • Solve for actual yield: